In many scientific disciplines, it is desirable to monitor the chemical reactions of a multiplicity of test samples. Such reactions can occur within specific time durations from a few seconds to days or weeks. One convenient and economical methodology is optical monitoring that can be applied if the chemical reactions modify certain optical characteristics of the test samples. For example, fermentation reactions can change the pH of a liquefied sample. If a pH color indicator is used, its color change can be detected by optical means as a function of time.
Many chemical reactions are associated with modification of the fluorescence characteristics of the test sample. Many fluorescence dyes have been developed for various chemical reactions. For example, Rhodamine-based compounds fluoresce when exposed to visible light radiation. Other compounds, such as Coumarines, fluoresce when exposed to long ultraviolet (UV) radiation. The advantage of the fluorescence compounds is that they are very sensitive and can provide early indication of specific occurring reactions.
The popularity of fluorescence monitoring has resulted in the introduction of special instruments which are based on fluorescence essays. Quite a few commercial instruments are available, based upon the following principles:                1. Single source single detector: This type is the traditional and most sensitive configuration. It usually utilizes a strong UV light source covering short UV bands, long UV bands and short visible bands (violet and blue). A UV filter to restrict the radiation to a specific UV wavelength is used. A single sensitive detector, such as a photo multiplying tube (PMT), combined with a visible light filter is utilized to detect the visible fluorescence light generated from the test sample due to the optical interaction of the sample with the light generated by the UV source. While this configuration is widely used due to its sensitivity, it has two major deficiencies. First, the light source, which is typically a strong discharge lamp, has a short life span—several hundreds hours—and has to be replaced often. Consequently, its price and maintenance prohibit its use for large scale automated processes, and therefore its usage is limited to few laboratory tests.        2. Indexed: To apply the above single source single detector configuration to multiple systems, a mechanical indexing means is utilized. Typically a micro-titer plate with multiple wells is indexed between the energy source and the detector (Bioscan Chamelton Multilabel Plate Reader). In modern systems, the UV light is conveyed from the light source via fiber optic cable. The emitted light can also be conveyed via another fiber optic line to the detector. With this configuration, multiple samples can be monitored, but the indexing means complicates the system and shortens its average failure time. The light source is still expensive, requiring frequent replacements.        3. Ultraviolet light emitting diode (UV LED): With the introduction of UV LED, some commercial systems (Turner BioSystems TBS-380) are available. Typically, a photo diode or PMT is used to detect the visible fluorescing light. With this configuration, multiple LEDs and photo detectors can be used for multiple tests. The main disadvantage of this configuration is the wavelength of the UV LED, which borders the visible light range (380-400 nanometers). For many reactions, this wavelength is not sufficiently short, yielding low fluorescence output.        